Audio waveform for digital recording

ABSTRACT

Digital information is entered, stored on and read off of transmission channels containing audio information. The digital information is stored in the form of discrete audible tone bursts representative of the state of each bit of digital information. Subaudible tone bursts are utilized for keying associated circuitry.

United States Patent 1 Schmitz et al.

[ AUDIO WAVEFORM FOR DIGITAL RECORDING Filed: July so, 1971 Appl. No.: 167,582

U.S. CI. 340/1741 11 Int. Cl. G1 lb 5/02 Field of Search 340/174.1 G, 174.1 H; 179/1002 S, 100.2 vB, 100.2 MD, 100.3 D,

References Cited UNITED STATES PATENTS 6/1966 Gabor 179/1002 S DlGlTAL BIT l7 SELECTOR I I8 7 x o o o o 0 so??? ooooo DATA ENTRY SWITCHES COUNTER DlGlTAL MULTI PLEXER 1 Sept. 18, 1973 3,222,597 12/1965 Bearenbough et a1 179/1002 S 3,571,526 3/1971 Stockwell 179/1002 S 3,334,194 8/1967 Chang 179/1002 S 3,524,026 8/1970 Langendorf et a1. 179/1002 S 2,780,679 2/1957 Vandivere, Jr. 179/1002 S Primary Examiner-Vincent P. Canney AttorneyF. H. Henson et al.

[57] ABSTRACT Digital information'is entered, stored on and read off of transmission channels containing audio information. The digital information is stored in the form of discrete audible tone bursts representative of the state of each bit of digital information. Subaudible tone bursts are utilized for keying associated circuitry.

10 Claims, 6 Drawing Figures DlGlTAL MU LTI PLEXER YOHZ I l OSCILLATOR "l cn sg sr ANALOG ANALOG MULTIPLEXER swn'c I4 20 8 ZERO CROSS BET. 2-s KHZ TO B OSCILLATOR 900 PHASE Is SHIFT a ZERO LATCH CROSS DETI LISTENER SPEAKER Q ss A 1 TONE CODE TAPE TONE CODE GENERATOR RECORDER READER FIG. I

. 9O ZERO PHA $E AND CROSS LATCH 5., SH! T DET FREQ SELECT 8 msec =0 l6 msec- I SOOmsec I f 1 1 40Hz I2kHz13kHzI2kHzI3kHz [2km 13kHzI2kHz13kHz [2km] 8 FIG.5

AUDIO WAVEFORM FOR DIGITAL RECORDING BACKGROUND OF THE INVENTION There are numerous techniques for recording digital information on audio tracks, be they magnetic or optical. The most promising technique when dealing with cassette type tape cartridges and optical audio tracks, is the simple frequency shift digital signal. However, if the frequency shift is implementedin the clearly audible range, the digital information, when intermixed with an audio message, results in a very distracting listening experience fora listener. However, if the frequency shift is implemented in the subaudible range, i.e., -50 Hz, all but a few bits of the digital information would take an unreasonably long time to be reproduced. The utilization of frequencies above the audible range likewise poses problems in that such frequencies are often unrecordable.

SUMMARY OF THE INVENTION A technique for producing an audio waveform consisting of subaudible and audible tones has been devised to store digital informationon audio (optical or magnetic) media. A short burst of subaudible tone, i.e. 40 Hz, is used as a keying signal which is used to mute the audio during code reading and which preconditions v the audio circuitry to read the digital information. The digital information consists of repetitive bursts of tones of different frequencies, i.e. 2 and- 3 His.

to a logic ZERO and a second predetermined duration,

i.e. 16 milliseconds, corresponds to a logic I. The selection of the series of 2 and 3 KI-Iz tone bursts to comprise the data code was based on the fact that these frequencies are in the center of the audio pass band and therefore can be very reliably recorded and reproduced. A 2 KHz tone burst occurs after each 3 KHz tone burst and'in essence serves as spacing between the digital data bits represented by the 3 KI-Iz tone bursts.

In the embodiment illustrated, a Hz keytone, or keying signal,.precedes each recorded data code. This 40 Hz tone is used to signal the code reading circuitry that acode will follow. The-40rl-Iz tone is below the normal'audible audio range but still can be recorded on the audio track. The 40.I-Iz tone functions to'blank the speaker audio during the time a code is being received thereby isolating the listenerfrom the annoying-2 and 3 Kl-lz tones.

Atypical use of this invention is in a studentlisteners recorded lecture material for providing-code information to transfer between predetermined recorded material.

DESCRIPTION OF THE DRAWINGS FIG. 4 is a detailed schematic illustration of a two frequency, single oscillator circuit incorporated in the code generator of FIG. 2; 1 I

FIG. Sis an illustration of the audio code tone generated by the system of FIG. 1; and

FIG. 6 is a block diagram illustration of the tone code reader of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1 there is illustrated in block diagram form a data encoding/decoding system comprising a tone code generator A, a data transmission channel in the form of a tape recorder B, and a tone code reader C. The tape recorder B is illustrated to represent an audio channel for linking the tone code generator A and the tone code reader C. A preferred embodiment of the system illustrated in FIG. 1 is that of an educational-instructional system. A detailed description of such an educational-instructional system is presented in the cofiled, copending application Ser. No. 167,581 entitled Programmed Instructional System which is assigned to the assignee of the present invention.

The tone code generator A of FIG. 1 is illustrated in block diagram form in FIG. 2. Data is entered into the tone code generator Aby means of data entry switches 1, which may take the form of a listener operated keyboard. In the discussion of the operation of the system which follows the depressed state of the data entry switches 1 represents a logic 1 and a non-depressed state of the switches represents a logic 0. After entering the data through the actuation of data entry switches l a start switch 2 is actuated. The actuation of switch 2 functions to set bistable flip-flop circuit 2 which in turn results in an output from one-shot circuit 4. The oneshot circuits referred to herein are functionally classified monostable multivibrators. The output from oneshot circuit 4 serves as a control input to digital multiplexerv circuit 5 and as a data input to latching circuit 6 while the output condition of bistable flip-flop circuit 3 serves as an input to latching circuit 22. The latching circuits 6, 16 and 22, as typically illustrated schematically in latching circuit 6, are well known devices havingan outputwhich follows the input witha control input signal C at a logic 1. When the control input signal C changes from logic 1 to 0, the output, remains latched to the state of the input that existed when the control input signal was last a logic 1. The output logic states of bistable flip-flop 3 and one-shot circuit 4 are considered to be logic I as illustrated in-waveforms A i and B of FIG; 3. A'zero crossing detector 7 responds I to the zero crossing points of the 40' Hz output of the 40 Hz oscillator 8 by generating a series of pulses as illustrated in waveform C of FIG. 3 which are supplied as the latching control input C of latching circuit 6 and as an input to digital multiplexer circuit 21. As illustrated in waveform D of FIG. 3 latching circuit 6 responds to the presence of logic 1 input from the oneshot circuit 14 and the zero crossing circuit 7, by gener-.

ating a logic 1 output which functions, to set analog multiplex circuit 9 to a state wherein the 40 Hz output of the oscillator circuit 8 is transmitted to' analog switching circuit 5. A logic 1 outputfrom latchingcirsuit 22 responds to the occurrence of a logic 1 input to, latching circuit 22 from flip-flop circuit 3 and a logic 1 input from digital multiplexer 21 as illustrated in waveform E of FIG. 3 to actuate analog switch circuit 21 to transmit the 40 cycle output signal generated by oscillator 8 to the tape recorder B. This output condition will persist until the one-shot circuit 4 times out. As illustrated in waveform D of FIG. 3, when the oneshot circuit 4 times out the output of latching circuit 6 changes from logic state 1 to logic state which in turn switches the analog multiplexer circuit A so as to terminate transmission of the 40 Hz output and to initiate transmission of the output of the 2 and 3 KHz oscillator 14. A change of state of the output of latching circuit 6 from logic 1 to logic 0 likewise functions to fire oneshot circuit 10. The firing of the one-shot circuit 10 allows a fixed period of 2 KHz tone to be applied to the analog output switch circuit 21. The gating of the 2 KHz tone by the analog switch 21 to tape recorder B is controlled by the output of the latching circuit 22. With the output of the one-shot circuit 4 at logic 0 the input supplied to the latching circuit 22 by digital multiplexer circuit 5 corresponds to the zero detection output pulses of 20 which corresponds to zero crossing points of the 2 and 3 KHz oscillator circuit 14. A change in signal supplied to the tape recorder B is illustrated in waveform M in response to the operation of the one-shot circuit illustrated in waveform F. When the one-shot circuit 10 times out, the clock circuit 11 begins oscillating as illustrated in waveform G of FIG. 3. At the time of the first clock pulse the output from the analog switch circuit 5 is a 2 KHZ tone since latching circuit 16 is at a logic 0 as illustrated in waveform I. An output from the clock circuit 11 results in termination of the 2 KHz output tone burst and initiation of a 3 KHz output tone burst from oscillator circuit 14. The 3 KHz tone burst is transmitted through analog multiplexer circuit 9 and analog switching circuit 5 to the tape recorder B. The period of the clock circuit 11 is controlled by the output of the digital multiplexer circuit 13. The output of the digital multiplexer circuit 13 is a function of shift register counter circuit 12 which reads the data entered on the data switches 10. If a data switch is depressed, the clock period is arbitrarily chosen to be 16 milliseconds and if the data switch is not depressed the clock period is arbitrarily designated to be 8 milliseconds. The data switches 10 are serially sampled by the shift register counter 12. When a data switch corresponding to a particular data bit it is depressed, corresponding to logic 1 a clock period of 16 milliseconds for that bit of the code is generated. If the data switch being sampled has not been depressed, corresponding to a logic 0, then a clock period of 8 milliseconds is generated. In order to prevent switching transients on the output of oscillator circuit 14, the output of latch 16, which controls the frequency of oscillator circuit 14 must change state at the peak voltage of oscillator circuit 14 or at a phase angle of 90. This is accomplished by phase shift and zero crossing detector circuit 15 which introduces a 90 phase shift and produces a pulse at the zero crossing of the phase shifted signal.

When the output of the clock circuit 1 1 changes from a logic 1 state to a logic 0 state, the phase comparator circuit 15 forces the latching circuit 16 to switch the output tone burst transmitted by oscillator circuit 14 from a 3 KHz tone burst to a 2 KHz tone burst. The 2 KHz tone burst will last for a period of 8 milliseconds. The tone code therefore can be defined as illustrated in FIG. 5 as comprising 8 or 16 milliseconds duration,

3 KHz information tone bursts followed by 2 KHz tone bursts of 8 milliseconds duration serving as spaces between the 3 KHz tone burst. The next clock pulse will result in a logic 1 output from the clock circuit 11. The counter 12 will count the next value entered in the data switches 10 through digital multiplexer circuit 13, the output of which will again control the length of the clock period and consequently the length of the 3 KHz tone burst transmitted by oscillator circuit 14 through the analog multiplexer circuit 9 and the analog switching circuit 21. This process will continue until the counter circuit 12 reaches a value equal to the reference value established by bit selector circuit 17. The bit selector circuit 17 permits selection of desired data codes containing 3, 8 or 10 bits. At this time comparator circuit 18 will cause the clock circuit 11 to stop counting and simultaneously trigger one-shot circuit 19. The bit selector circuit 17 determines the number of bits which will be generated in the code. The oneshot timer circuit 19 triggers bistable flip-flop circuit 3 and results in the transmission of a 2 KHz tone burst at the output of the analog switching circuit 21 as shown in waveforms J and K of FIG. 3. When the one-shot circuit 19 times out the bistable flip-flop circuit is turned off and the tone code generator A is returned to an idle condition when latching circuit 22 turns off analog switch 21 coincident with the output of zero cross detector circuit 20.

Referring to FIG. 5 it is noted that the 2 KHz tone following the 40 K2 key tone is longer than the 8 millisecond 2 KHz tones which separate the 3 KHz data tones. The additional time insures the decay of transients which may occur in the recording channel when the change is made from 40 Hz to 2 KHZ. The final 2 KHz tone of the code is made long so that the end of the data code can be detected by measuring the length of the 2 KHz tone.

The clock circuit 11, which is functionally identified as a two period clock, is comprised of a plurality of RC circuits such as that designated by resistor R and capacitor C. Resistor R is always in the timing circuit while resistor R1 is connected into the circuit to the operation of switch S. Insertion of resistor R1 in parallel with resistor R2 alters the period of clock circuit 11 as controlled by the output of thedigital multiplexer circuit 13.

The 2 and 3 KHz single oscillator circuit 14 is illustrated schematically in FIG.4. The oscillator circuit 14 is illustrated as being comprised of an amplifier A, a three section RC phase shift oscillator circuit when the switch S is in the open position, and a four section RC phase shift oscillator when the switch S is in the closed position. The output frequency of the oscillator circuit 14 is controlled by the switching of capacitor C4 in and out of the circuit in response to the output of the latching circuit 16. With the capacitor C4 out of the circuit the oscillator 14 will produce a 3 KHz tone while the insertion of capacitor C4 in the circuit by the closure of switch S will result in an oscillator output of 2 KHz. The output of the amplifier A is passed through phase shift and zero crossing detector circuit 15. The function of the phase shift and zero crossing detector circuit 15 is to trigger latching circuit 16 at the exact proper phase so that the oscillator 14 will switch from one frequency to the other frequency in a continuous manner. When the clock circuit 11 changes state, the latching circuit 16 does not change state and change the frequency of oscillator 14 until the phase of oscillator 14 is at 90 to produce a continuous waveform. The continuous waveform defines a smooth continuous transition in amplitude with a minimum change in slope one frequency to the next frequency thereby avoiding sudden changes or spikes in amplitude which could seriously affect the operation of the system.

Referring to FIG. 6 there is illustrated in block diagram form components comprising the tone code reader C of FIG. 1. The audio from the tape recorder B enters the tone code reader circuit C and is divided into two channels, a high frequency channel and a low frequency channel. The low frequency audio is transmitted through a 40 Hz filter 50, rectified by rectifier circuit 51 and smoothed by filter capacitor C2. The DC level produced at capacitor C1 is a measure of the magnitude of 40 Hz tone present at the input of the filter 50. When the DC level exceeds that of the reference V,,,,, the comparator circuit 53 produces an output signal which in turn latches the flip-flop circuit 64.

The high frequency audio is transmitted through a limiter circuit 53 which produces a square wave output even with very small input signals which is subsequently applied to current sources 54 and 55. The current sources 54 and 55 provide high impedance drive signals in order to prevent excessive loading of tuned filter circuits 56 and 57. The output of current source 54 is applied to a 2 KHz filter 56, the output of which is subsequently rectified by rectifier circuit 58, smoothed by capacitor C2 and applied as an input to comparator circuit 60. The output of the current source 55 is applied to a 3 KHz filter 57. The output of the filter is subsequently rectified by rectifier circuit 59 and smoothed by capacitor C3. The smoothed output of the rectifier 59 serves as a second input to the smoothed comparator circuit 60. The comparator circuit 60 decides which of the two input signals is larger. A logic 1 from the comparator circuit 60 corresponds to the presence of a 2 KHz signal while a logic 0 output from the comparator circuit 60 corresponds to the presence of a 3 KHz signal. The output of the comparator circuit 60 serves as a clock signal.

- The clock signal output of comparator circuit 60 and thetoutput from latching circuit 64 are combined in logic AND gate 62. The output of AND gate 62 is the clocking signal for shiftregister 63. At the transition from 3 KHz to 2 KHz the content of shift register 53 is shifted one spaceto the left.

Time comparator circuit 61 measures the' length of each 3 KHz tone. If the 3 KHz tone is shorter than a preselected time, i.'e. 12 milliseconds (a value between 8 and 16 milliseconds), then no, output occurs from comparator circuit 6l thereby indicating a logic 0 is present in the recording channel of tape recorder B for that particular data bit. If the 3 KB: tone is longer than 12 milliseconds,i.e. 16 milliseconds, then a logic 0 output'is produced by comparator circuit 61. This zero sets the first stageof shift register'63 to a 1. This 1 is shifted one position to the left in shift register 63 for each successive transition from 3 KHz to 2 KHz. Thus after the final 3 KHz tone is completed the outputs of the shift register 63 correspond identically to the data transmitted by the tone code generator A in response to actuation of the data switches 1.

Time comparator circuit 65 measures each 2 KHz tone burst and when the duration of the tone burst exceeds a preset value, Le. 30 milliseconds (a value greater than 8 but less than 50 milliseconds), the comparator circuit 65 resets flip-flop 64 thus disconnecting the clock pulses from shift register 63 and terminating the data retrieval cycle.

The output of flip-flop 64, after being set by the 40 Hz keying tone, causes analog switch 66 to open speaker switch 55 of FIG. 1 to prevent the audio from tape recorder B from being transmitted to listener speaker LS. This isolation of the listener speaker LS, which may also be represented as car phones, prevents the annoying 2 and 3 KHz tone bursts from reaching the listener. At the completion of the tone code pattern comprised of the 2 and 3 KHz tone bursts, the flip-flop 64 is reset by time comparator circuit 65 and the analog switch 66 functions to close speaker switch SS and return the audio to the listener speaker LS.

We claim: 1. Apparatus for combining subaudible and audible tones for storage of bits of digital information on an audio track, comprising, first means for generating a subaudible tone burst of a predetermined duration for recording on said audio track, second means for logically representing said bits of digital information for storage on said audio track following said subaudible tone burst, third means including a multiple frequency oscillator circuit means operatively connected to said second means for responding to a first output condition of said second means by generating an audible tone code pattern comprised of spaced apart tone bursts indicative of said bits of digital information for storage on said audio track, and responding to a second output condition of said second means by generating audible tone bursts of asecond frequency to serve as spaces between said spaced apart tone bursts.

2. Apparatus as claimed in claim 1 further including fifthmeans for providing a relatively uninterrupted, continuous transition between the audibletone bursts of the different frequencies.-

3. Audio reproduction apparatus, comprising, a multitrack storage media having a plurality of segments of audio information recorded on one or more tracks and including tone code pattemsassociated with said segments of audio information, each tone code pattern consisting of a subaudible tone followed by an audible tone code pattern corresponding to bits of digital information, said audible tone code pattern identifying the location of the subsequent audioinformation to be reproduced, I

first means operatively connected to said multitrack storage media for presenting said audio information to a listener,

second means responding to said subaudible tone to deactivate said first means prior to reproduction of said audible tone code pattern,

third means responding to said audible tone code pattern to cause reproduction of audio information at a track location defined by the digital information content of said audible tone code pattern, and fourth means responding to the conclusion of said andible tone code pattern by activating said first means prior to reproduction of the audio informa tion.

4. Apparatus as claimed in claim 3 wherein said audible tone code pattern is comprised of discrete audible tone bursts.

5. Apparatus as claimed in claim 3 wherein said audible tone code pattern is comprised of spaced apart audible tone bursts.

6. Apparatus as claimed in claim 5 wherein each tone burst corresponds to a bit of digital information and the length of each tone burst is indicative of the state of said bit.

7. Apparatus as claimed in claim 3, wherein said audible tone code pattern is comprised of alternate first and second tone bursts of a first and second frequency, respectively, thelength of said first tone bursts being indicative of the state of the bits of digital information, said second tone burst providing spacing between each bit of digital information.

8. Apparatus as claimed in claim 7, wherein first tone bursts of a first predetermined length correspond to a logic 1 and first tone bursts of a second predetermined length correspond to a logic 0.

9. Apparatus as claimed in claim 7, wherein the initial and final tone code bursts of said audible tone code pattern is a second tone burst of a duration longer than the remaining second tone bursts comprising said audible tone code pattern, said remaining second tone bursts being substantially identical.

10. Apparatus as claimed in claim 7, wherein the frequency of said key tone is approximately 40 Hz, the frequency of said first tone bursts is approximately 3 kHz, and the frequency of said second tone bursts is approximately 2 KHz. 

1. Apparatus for combining subaudible and audible tones for storage of bits of digital information on an audio track, comprising, first means for generating a subaudible tone burst of a predetermined duration for recording on said audio track, second means for logically representing said bits of digital information for storage on said audio track following said subaudible tone burst, third means including a multiple frequency oscillator circuit means operatively connected to said second means for responding to a first output condition of said second means by generating an audible tone code pattern comprised of spaced apart tone bursts indicative of said bits of digital information for storage on said audio track, and responding to a second output condition of said second means by generating audible tone bursts of a second frequency to serve as spaces between said spaced apart tone bursts.
 2. Apparatus as claimed in claim 1 further including fifth means for providing a relatively uninterrupted, continuous transition between the audible tone bursts of the different frequencies.
 3. Audio reproduction apparatus, comprising, a multitrack storage media having a plurality of segments of audio information recorded on one or more tracks and including tone code patterns associated with said segments of audio information, each tone code pattern consisting of a subaudible tone followed by an audible tone code pattern corresponding to bits of digital information, said audible tone code pattern identifying the location of the subsequent audio information to be reproduced, first means operatively connected to said multitrack storage media for presenting said audio information to a listener, second means responding to said subaudible tone to deactivate said first means prior to reproduction of said audible tone code pattern, third means responding to said audible tone code pattern to cause reproduction of audio information at a track location defined by the digital information content of said audible tone code pattern, and fourth means responding to the conclusion of said audible tone code pattern by activating said first means prior to reproduction of the audio information.
 4. Apparatus as claimed in claim 3 wherein said audible tone code pattern is comprised of discrete audible tone bursts.
 5. Apparatus as claimed in claim 3 wherein said audible tone code pattern is comprised of spaced apart audible tone bursts.
 6. Apparatus as claimed in claim 5 wherein each tone burst corresponds to a bit of digital information and the length of each tone burst is indicative of the state of said bit.
 7. Apparatus as claimed in claim 3, wherein said audible tone code pattern is comprised of alternate first and second tone bursts of a first and second frequency, respectively, thelength of said first tone bursts being indicative of the state of the bits of digital information, said second tone burst providing spacing between each bit of digital information.
 8. Apparatus as claimed in claim 7, wherein first tone bursts of a first predetermined length correspond to a logic 1 and first tone bursts of a second predetermined length correspond to a logic
 0. 9. Apparatus as claimed in claim 7, wherein the initial and final tone code bursts of said audible tone code pattern is a second tone burst of a duration longer than the remaining second tone bursts comprising said audible tone code pattern, said remaining second tone bursts being substantially identical.
 10. Apparatus as claimed in claim 7, wherein the frequency of said key tone is approximately 40 Hz, the frequency of said first tone bursts is approximately 3 kHz, and the frequency of said second tone bursts is approximately 2 KHz. 